FIELD OF THE INVENTION
This invention describes xerogels and a process for their preparation. These xerogels have a higher porosity, which gives these compositions characteristics more similar to aerogels.
It is well known that gels can be dried to obtain porous, low density materials. Compositions with such features are particularly attractive for insulation and as matrices for radioluminescent light sources. Applications could also be found in acoustics, catalysis, filtration, high energy physics, solar energy and conservation.
Xerogels are dried gels which are formed by drying the gel at atmospheric pressure, typically using elevated temperatures; shrinkage of the gel's solid network occurs in drying. Unfortunately, in fact, the resulting xerogel is often reduced in volume by a factor of 5 to 10 compared to the original wet gel. The undesired loss of volume has in fact been noted in The Concise Chemical and Technical Dictionary (Chemical Publishing Co. 1974) which teaches that the removal of volatile fluid from the gel during the xerogel preparation results in shrinkage to a harder and stronger mass. That shrinkage is disadvantageous for properties such as porosity, density and thermal conductivity, which are all important for insulation.
Low-density xerogels are discussed in D. M. Smith, Better Ceramics through Chemistry V, Mat. Res. Soc. Symp. Proc. Vol. 271, pages 567-572 (1992). According to this reference, aging and surface derivatization can be used to control the capillary pressure and gel matrix strength of base-catalyzed alkoxide derived silica gels and subsequent formation of xerogels under ambient pressure conditions. Generally, the removal of pore fluids during drying in a non-supercritical process causes the gel network to collapse due to the high capillary pressure and further chemical reactions between surface sites.
Aerogels are more highly porous and less dense but they are produced by drying the gel under supercritical conditions of pressure and temperature where a single phase exists (note U.S. Pat. No. 2,249,767). Other references which discuss aerogels and their preparation include: U.S. Pat. Nos. 2,978,298; 3,977,993; 4,402,927 and L. W. Hrubesh, Chemistry and Industry, 24 824, 1990. Supercritical processes, however, are costly, potentially dangerous, and more complex, which makes these processes impractical, particularly in large scale operations.
Aerogels produced by drying wet gels under the supercritical conditions which maintain a single phase (allowing no interface between liquid and vapor) also are, disadvantageously, very difficult to make, although their properties and characteristics are more desirable than xerogels. Under supercritical conditions there is minimal shrinkage and the aerogels have a low volume fraction of solid.
Xerogels are made at atmospheric pressure. It would, therefore, be advantageous to have lower density, more highly porous xerogels. Most desirably the porosity should approach the porosity of aerogels. A non-supercritical process for the production of such xerogels is needed.
As an object of the present invention xerogels are produced which have a lower density and a higher porosity.
A further object of the present invention provides a method of chemically treating a wet gel with silicon-nitrogen compounds to reduce the compressive forces and shrinkage when drying under non-supercritical conditions. The use of this chemical treatment greatly reduces the cost by eliminating the need to contain the gel under high pressures.
This invention also provides xerogels which are hydrophobic and therefore are not attacked by moisture.
The instant invention, furthermore, achieves this xerogel modification using non-hazardous reagents and by-products.